Verse 8.6.6

तदेष श्लोकः । शतं चैका च हृदयस्य नाड्यस्तासां मूर्धानमभिनिःसृतैका । तयोर्ध्वमायन्नमृतत्वमेति विष्वङ्ङन्या उत्क्रमणे भवन्त्युत्क्रमणे भवन्ति ॥ ८.६.६ ॥
॥ इति षष्ठः खण्डः ॥

tadeṣa ślokaḥ | śataṃ caikā ca hṛdayasya nāḍyastāsāṃ mūrdhānamabhiniḥsṛtaikā | tayordhvamāyannamṛtatvameti viṣvaṅṅanyā utkramaṇe bhavantyutkramaṇe bhavanti || 8.6.6 ||
|| iti ṣaṣṭhaḥ khaṇḍaḥ ||

6. There is a verse about this: There are a hundred and one arteries connected with the heart. One of them goes up to the top of the head. A person who goes up following this artery attains immortality. The other arteries go in different directions and cause one to depart from the body in other ways.

Word-for-word explanation:

Tat eṣaḥ ślokaḥ, there is a verse about this; śatam ca ekā ca, one hundred and one; hṛdayasya nāḍyaḥ, arteries of the heart; tāsām ekā, one of them; mūrdhānam abhiniḥsṛtā, extends to the top of the head; tayā, by that [artery]; urdhvam āyan, going up; amṛtatvam eti, one attains immortality; anyāḥ, as regards the others; viṣvan utkramaṇe bhavanti utkramaṇe bhavanti, they cause one to depart from the body in different directions, they cause one to depart. Iti ṣaṣṭhaḥ khaṇḍaḥ, here ends the sixth section.

Commentary:

The Upaniṣad says, there are a hundred and one arteries, and one of these goes up to the crown of the head. That is the sūrya nāḍī, the passage to the sun, described in the previous verse. It goes from the heart to the top of the head. Those who depart from the body through this passage attain immortality—that is, liberation.

But what happens to other people? Those who depart through other passages must be reborn. They may be reborn as a human being, or they may be reborn as an animal, or even as an insect. But please note: It is the same Self that is a human being, an animal, and an insect. From an amoeba to the state of Brahman, it is the same Consciousness.

You may start as an amoeba, never mind. If you are Brahman, you are always Brahman, whether you have a good body or not. You may not know you are Brahman. But that does not change the fact that you are Brahman. You may think you are just an amoeba, because you identify yourself with an amoeba body. Similarly, you may think you are a dog because you have a dog body. Or you may think you are a human being because you are in a human body. But this is all ignorance. Your goal is to realize yourself as Brahman, and, no matter what kind of body you have now, some day you will reach that goal.